Thin, planate articles such as wafers and discs are economically handled by merely disposing one article horizontally upon another in a vertical column. However, if the articles are fragile, if the surfaces are vulnerable to scratches or if ready access to individual articles is desired, then different expedients are indicated. Such expedients may include a vertical or horizontal column of articles held in a succession of individual pockets, often referred to as a "file." Normally, if such a file of articles is to be carried about, the articles are each vertically oriented in a mutually spaced, parallel relationship along a single, horizontal file in a carrier.
Such carriers are particularly evident in the semiconductor industry where disc-shaped wafers, typically of monocrystalline silicon, form a basic medium for making electronic devices. Such wafers may be about 100 mm in diameter by about 0.5 mm in thickness and weigh about 15-20 grams. Prior to forming devices therein, the wafers are ground and polished and often edge contoured to minimize chipping. Thereafter, several hundred discrete steps may be taken to form about 200 circuits in such a wafer and each circuit may include 60,000 or more microelectronic devices. It will be appreciated that such wafers take on considerable value as they are processed; yet they are fragile, their features are delicate and they are readily damaged. Consequently, great care is required to avoid chipping the edges or scratching and abrading, particularly on active surfaces containing the microminiature devices.
One popular carrier for handling such wafers is about 140 mm long by about 125 mm wide by about 115 mm high and accommodates 25 wafers, each in a separate pocket. The carrier has a fully accessible top, partially open bottom and ends and slotted sidewalls. Accordingly, there is considerable access to the wafers for process solution and drainage or for loading and unloading wafers. The sidewalls have lower portions which are offset inwardly to support along the curve of a wafer, engaging only edge portions thereof. The sidewalls also have opposing guides, forming therebetween mutually spaced, substantially parallel pockets for retaining wafers in a separated manner.
Such a carrier is suitable for most process steps so the wafers need not be repetitively transferred to and from other holders. However, for very hot operations such as solid state diffusion, the wafers are transferred to different holders and such transfer is fraught with difficulty. Also, the wafers are spaced so close in the described carrier that it is difficult to remove a randomly selected wafer for inspection or testing without damage to it or to adjacent wafers.
One way to transfer the wafers is to equip a receiving holder with pockets corresponding to those of the carrier. Then the holder may be mated to the carrier and both inverted whereby the wafers slide from carrier to holder. Another batch method is to carefully stand a filled carrier on end and push the wafers from carrier to holder pockets. Another common method of transfer is to utilize a pair of tweezers especially made to grasp narrow, marginal surfaces of a wafer. These and other techniques invite a risk of frequent damage due to sudden stops, frictional debris and grasping stresses which cause edge chipping and surface abrasion.
An improved technique for selecting wafers from a close array includes a mechanism for displacing at least one wafer at a time upwardly in a pocket. Then a vacuum tool is advantageously applied to a minimally sensitive, inactive surface and a wafer is manipulated thereby with little risk of damage to it or to adjacent wafers. Such selection mechanisms generally function by a series of levers and return springs and are awkward in operation. Moreover, the mechanisms are complicated and costly to provide.
Consequently, it is desirable to provide new and improved expedients for selecting thin, planate articles from an array thereof. It is preferred to select disc-shaped wafers by displacing selected random ones at least partially out of pockets in a carrier. Alternatively, it is preferred to displace select groups, for example every third wafer, from normal rest positions along a single file in a carrier.